Enhance Container Connection Env Variables Documentation

[kachawla]

What content needs to be created or modified?

We currently have documentation on the format of environment variables that are injected for connections from Radius container: https://docs.radapp.io/guides/author-apps/containers/overview/#connections. However, it lacks information on how we store these environment variables securely.

Describe the solution you'd like

We should document that we use secret to store the values https://github.com/radius-project/radius/blob/main/pkg/corerp/renderers/container/render.go#L722, https://kubernetes.io/docs/concepts/configuration/secret/#using-secrets-as-environment-variables.

Where should the new material be placed?

https://docs.radapp.io/guides/author-apps/containers/overview/#connections

The associated pull request from the Radius code repo, if applicable

Not a pull request, but existing relevant code is here: https://github.com/radius-project/radius/blob/main/pkg/corerp/renderers/container/render.go#L722

AB#10599